On the housing where the thermostat bolts on, just above the water pump and below the upper rad line (the suppily), there is a temperature sensor. Remove it, install a Tee into the fitting and reinstall the temp sensor in one of the legs of the Tee, and the sensor from the after-market gauge in the other. run the sensor wire through the firewall and up to the gauge in the dash. Connect the lights to the gauge and there you go. On my 4cyl there was another temperature sensor at the back of the head (up close to the firewall) that went to the dash idiot light. I suppose you could pull it, and install the sensor there. But I kinda liked having the idiot light as a backup. The sensor in the front that you are teeing into is the one that measures engine temp for the computer, so make sure that you reinstall it.
If you don't have a temp. light, or temp gauge, then just install one.
Take it to a mechanic
If the guage itself is broken it will have to be replaced, possibly part of the dash cluster. Might be easier to install an aftermarket guage in the dash and don't worry about the original guage. Aftermarket guages are better as they actually give you a temperature number instead of a "normal" area on the guage and you have to guess what it actually is.
It all depends on the rating of your thermostat which is usually 180 to 195 degrees if using a aftermarket temperature gauge and headers the temp. gauge reading will be higher than normal because of the excessive heat from the headers.
why doesn't my rpm gauge and temp gauge, fuel gauge not work
Look for a manufacture that supplies a mechanical gauge and check directions for location it depends what the size fittings they use.
1996 prelude temperature gauge reads low temp
temp gauge is stuck on hot
Depending on which intake manifold you have there may be a plug in it near the thermostat housing. Also, each head will have a plug on the side. One is usually fitted with the sensor for the original gauge. This is the usual spot for wiring in an aftermarket gauge.
only 3 reasons a temp gauge wont work the temp sender is bad the gauge is bad or the wire from temp sender to gauge is broken
There are two temp sensors on that year engine on for the gauge which is screwed into the head near the exhaust manifold and the other one is for the idiot light it is near the thermostat.
My Corsica's temp gauge went out, and to replace the OEM gauge would have required buying the whole instrument panel. We bought an aftermarket gauge and mounted it under the dash. It cost much less, and hey, it's not a show car so I don't mind that it's not 'perfect'. Just find the temp sensor on the engine (driver's side of car on back side of engine), and either use the sensor wire there or replace with one that works with your gauge.